Sql Common Interview Questions

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This article will provide a list of some common SQL interview questions that, along with the answers, you can find in an actual interview.

To get the most out of it, try to answer each question yourself. 1. What is SQL?

MySQL, which means built-in query language, is a programming language for communicating with data stored in relative database management systems.

Its syntax is similar to English, which makes it easier to write, read and translate. 2. What is a database?

A database is a set of data stored in a computer, where the data is structured in such a way that it is easily accessible. 3. What is a relational database?

A relative database is a type of database.

The data in a relative database is often organized as a table.

4. What is a RDBMS?

A relative database management system (RDBMS) is a program that allows you to create, update and manage relative databases.

Most relative database management systems use MySQL language to access databases.

The most popular RDBMS is MySQL. 5. What is a table?

A table is a collection of data divided into rows and columns.

They can contain hundreds, thousands, and sometimes even millions of rows of data.

6. What is a row and column in a table?

A row is the only record of data in a table.

A column is a set of data values ​​of a particular type.

7. What is a data type?

Data type is an attribute that defines the type of data in the column.

Each column in the database has a data type.

Some common data types are INTEGER , A basic key is a column that identifies each row of a table separately.

Primary keys must meet the following requirements: No value can be NULL , no more than one key in each table.

For example, Users In the table, the basic key >>.

A foreign key is the basic key of a table that appears in a different table. 9. What is the difference between ALTER and UPDATE?

ALTER is used to add a new column to the statement table.

Update Used to edit a row in the statement table. 10. What is a query?

A query is a SQL statement used to retrieve information stored in a database.

They allow us to interact with the database by asking questions and returning a result set with the data associated with the query.

11. What is a subquery?

A subfolder is an internal query that resides within an external query.

they select , <<

When a subdomain exists, it will be executed before running an external statement.

12. What are constraints?

Obstacles are a set of rules used to define a database to limit the type of data stored for columns.

They include information about how the column can be used, and are requested after typing data for the column.

Here are some examples of barriers:

Basic key >, which identifies each row individually and the value of each needs to be individualized. 13. What is a statement?

is a statement text that the database recognizes as a valid command.

They can be used to perform tasks such as changing the structure of a table, updating data, or retrieving data from a database.

The structure of a statement may be different, but each should end in a semi-semicolon. 14. How do you check if a field does not have a value or has a value?

When a field has no value, it is indicated by the NULL value.

To test if a field has no value, you can use IS NULL as a condition: WHW

To check if a field has a value, you need to not its status as a condition: DISTINCT A keyword is used when we want to return unique values ​​to the output.

UNIQUE is a barrier used to ensure that all column values ​​are different. 16. What are aggregate functions used for?

Aggregate functions are used to calculate one or more values, and get a value for more meaningful information.

There are some aggregate functions. 17. What is a join?

Adding is a way to combine rows from two or more tables, based on the corresponding columns between them.

18. What is the difference between an INNER JOIN and LEFT JOIN?

A INNER JOIN is used to add common rows from two tables that have 7 indicates.

A left table is used to place all rows from the first table, regardless of whether the << code for the second Whether there is a matching row in the table 19. What is the purpose of windows functions?

Windows functions are used when you want to retain the values ​​of your original table while displaying group or aggregate information. 20. What are indexes and why are they needed?

Indexes are used in the background of a database to speed up the query, acting as a flexible table of databases.

They need to store data efficiently for faster retrieval, which can be critical to the success of large tech companies that execute data on a per-byte byte scale every day. need to.

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